A resid hydrotreating unit (system) has a series of ebullated bed reactors for processing a resid oil stream, also known as vacuum-reduced crude, residual oil, or unhydrotreated virgin resid. In each reactor, the resid is hydroprocessed (hydrocracked and hydrotreated) in the presence of hydrogen and of a fresh or equilibrium hydrotreating catalyst in order to produce an upgraded effluent product stream. Fresh hydrotreating catalyst may be fed downwardly into the top of the reactor while partially spent catalyst may be withdrawn from the bed of the bottom of the reactor to maintain catalyst inventory constant. A hot resid feed and hydrogen-containing feed gases enter through feed lines in the bottom of the reactor. The entering resid and hydrogen separate within a plenum chamber positioned in the interior of the bottom portion of the ebullated bed reactor.
The oil and gas feed are re-mixed and blended in a homogeneous manner in the slotted risers of the bubble caps. The mixture is distributed over the bottom of a catalyst bed through the bubble caps in order to provide a uniform upward flow pattern through the bed, the upward flow being with enough force to ebullate and expand the catalyst bed. To provide this pattern and insure a uniformity of the homogeneous mixture of oil and gases which flows upwardly, a grid plate is positioned over the oil in the plenum and the catalyst bed in order to help distribute the oil and gas across the cross section of the reactor. The grid also prevents the catalyst from falling into the plenum at the bottom of the reactor. An ebullating pump circulates the liquid, vapor, oil and gas mixture at a rate which is sufficient to lift and expand the catalyst bed from its initial settled level to its steady state expanded and ebullating bed level.
An ebullated or expanded bed reactor relies on a uniform distribution of proper balance between liquid and gas (a foam-like mixture) to maintain a uniformly expanded catalyst bed. The combination of liquid and gas is recycled oil, new feed oil, and hydrogen.
Each piece of the catalyst is a grain which is in the order of 1/16 to 1/8-inch in diameter more or less. Ideally the catalyst bed would expand so uniformly that each and every catalyst grain would be exactly the same distance from all of its neighboring grains. Then, the fluid being processed would bathe the surface of each and every grain in exactly the same manner so that a maximum efficiency of catalyst usage would be obtained.
This idealized expansion of the catalyst does not occur; however, every effort should be made to approach the ideal. Among other things, this means that the mixture of fluid and gas (a foam) in the plenum should enter the catalyst bed as uniformly as possible across the entire lower cross sectional surface of the bed. For example, if the entering fluid should be maldistributed so that more fluid enters, say, the north side of the catalyst bed, as compared to the fluid entering the south side of the bed, any of a great many different adverse effects could be encountered. Convection currents could be set up within the expanded catalyst bed to deflect the stream of processing oil through the bed in some unpredictable manner. One side of the catalyst bed could slump while the other side expands. Coke could form on one side of the bed. Passageways into and through the bed could clog on one side and channels could open on the other side.
In order to properly distribute the fluid and in an effort toward uniformity of the catalytic bed, the fluid entering the catalyst bed is fed through the grid plate which has many holes formed therein and distributed there across. This grid plate arrangement restricts the fluid entering the catalyst bed via the holes which builds a pressure differential (about 3-5 psi) on opposite sides of the grid plate. The pressure differential drives the entering fluid with enough force so that, in theory, a uniform amount of fluid is forced to pass through each grid plate hole depending upon the area of the hole. However, turbulences may build up around the hole to deflect the fluid passing through it.
Among other things, this pressure differential, the restrictions at the surface of the grid plate, and the like causes a blanket of vapor to develop between the grid plate and an underlying surface of liquid oil beneath the plate. This blanket of vapor and oil interface level is established somewhere between the bottom of the riser and the base of the slot in the riser which allows a very uniform distribution of gas and liquid through the risers. However, if the vapor blanket is lost, through a leak in the grid, all the liquid goes through the risers and the vapor by-passes the bubble caps with uneven flow through the various holes in the grid and the above noted and other problems begin to appear in the ebullated bed.
In an effort to maintain the integrity of the vapor blanket, and to avoid the maldistribution at the holes, it has been common practice to extend risers (short pipes) downwardly from each grid plate hole far enough (about 8 to 12 inches) below the underside of the grid to pass through a minimum thickness of the vapor blanket and to be below any turbulent area within the blanket. The length of these riser pipes tends to insure both the continued existence of the vapor blanket and the uniformity of liquid distribution within the catalyst bed. However, the thickness of the blanket and the height of the upper surface level of the liquid tends to fluctuate. Therefore, the bottoms of the riser pipes contains one or more vertical slots which are long enough so that if a raising level of the oil surface should reach the bottom of the riser pipe, the vapor may still enter the riser via the slots.
Ideally, the periphery of the grid plate should be sealed (welded, or the like) to the inside of the reactor housing. However, the thermal expansions and contractions which occur during reactor operation would cause severe stress problems so that welding or another form of sealing is not a very practical option. Therefore, at its periphery, the grid plate is bolted into position to enable it to expand and contract with temperature changes within the reactor. This mode of attachment may lead to cracks or other leakage at or near the periphery of the grid plate. Once even a small leak is formed, the pressure differential on opposite sides of the grid plate disappears. Without a pressure differential, the vapor blanket disappears and liquid begins streaming through the holes in the grid plate.
Because of the relatively large pressure drop across the grid (typically 3-5 psi), velocities through a grid leak can be several hundred feet-per-second. These high vapor velocities could cause the catalyst to elutriate out of the top of the reactor and through the ebullating pump. Once catalyst begins elutriating from the top of the reactor the catalyst may be recycled with a slurry through the ebullating pump causing the catalyst to break up and wear away.
In one case where there was severe elutriating, thermocouples located above the grid plate and within the reactor indicated that most of the flow through the catalyst bed was directed toward the west side and only a little flow was going up the east side of the reactor. Toward the end of the period, one skin thermocouple on the east side registered 830.degree. F. as compared to the normal range of 500.degree.-600.degree. F. for the refractory-lined reactor. Due to a flow maldistribution, catalyst attrition, and the hot spot on the east wall, the plant was shut down and the reactor was opened for inspection.
An inspection of the reactor revealed a 12".times.8".times.24" mass of hard coke on the east wall above the grid. Moreover, thirty-eight of the ninety-six grid holes were plugged, all on the east side. Finally, large grid leaks were present on the NNW and SSW locations at the grid support ring. Thus, while reactors equipped with grid plates having risers connected therewith provide for the upward movement of an evenly distributed liquid-vapor mixture, it does not address the problems created by one or more grid plate leaks.
U.S. Pat. No. 2,836,902 ('902) discloses a grid and grid sealing arrangement comprising a skirt-like apparatus for supporting the fluid bed within a burning zone and for uniformly distributing combustion air throughout a bed in, for example, the fluid coking process used in the petroleum industry. However, the '902 skirt was developed and specifically designed to allow differences in thermal expansion and contraction in the equipment, while providing support for the grid and preventing passage of gases around the periphery of the grid.
U.S. Pat. Nos. 4,715,996 and 4,753,721 assigned to Amoco Corporation, disclose a bubble cap assembly and feed distributor which are particularly useful for resid hydrotreating.